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The 30-year life requirement of the Boeing Mod II windmill results in a service life load spectrum having an excess of 200 million cycles. Determination of allowable stress levels for the type of steels being used in construction of the systems, requires a detailed knowledge of their crack growth behavior under representative spectrum. A spectrum load test program was conducted, using uniaxially loaded surface flaw specimens, to provide the data needed to ascertain the impact of differences in load spectra on crack growth behavior.
A total of four different load spectra were tested to provide enough diversity in test parameters to ensure that the actual load spectra for the system would be enveloped by those tested. The test results were analyzed using different crack growth models to determine which produced the best correlation between actual and predicted results. Excellent correlation between predicted and actual test results was obtained when a crack growth model containing both a threshold and retardation term was used. This model can be used to estimate the spectrum load crack growth behavior of steels covered by the ASTM Specification for General Requirements for Rolled Steel Plates, Shapes, Sheet Piling, and Bars for Structural Use (A 6–76a) and Specification for General Requirements for Steel Plates for Pressure Vessels (A 20–76b).
fatigue (materials), spectrum load, crack growth, steels, windmills
Specialist engineer, Boeing Engineering Company, Seattle, Wash.